This invention relates to optical retarder devices, and particularly to fast response liquid crystal cell optical retarder systems and polarization control systems.
In optical devices, instruments, communication systems and laboratory set-ups, it is often desirable to be able to selectively vary the retardance of one polarization component of a light beam relative to another, orthogonal component so as to vary the total polarization of a light beam. This has typically been accomplished using optical wave plates disposed parallel to one another wherein the polarization is varied by rotating the fast axis of one wave plate with respect to the fast axis of the other. In fiber-optic communication systems, polarization adjustability has been accomplished typically by mechanical devices that squeeze, or otherwise stress the fiber, so as to change its birefringent properties.
Polarization can also be selectively controlled by the use of a liquid crystal cell retarder. In such devices, the phase of light polarized along one axis with respect to another, orthogonal axis varies in accordance with the amplitude of an applied ac voltage. This characteristic has been employed in optical shutters, as disclosed in Box U.S. Pat. No. 4,635,051, issued Jan. 6, 1987 and entitled "High-Speed Electro-Optical Light Gate and Field Sequential Full Color Display System Incorporating Same," and in polarization control systems, as disclosed in Rumbaugh U.S. Pat. No. 4,979,235, et al. issued Dec. 18, 1990 and entitled "Polarization Controller for Use in Optical Fiber Communications System" and Clark U.S. Pat. No. 5,005,952, et al issued Apr. 9, 1991 and entitled "Polarization Controller," all of which patents are herein incorporated by reference in their entirety. However, where used to vary polarization by switching between intermediate values over a range of retardances, known liquid crystal cell retarder systems have two significant drawbacks.
First, a change in retardance in one direction must be effectuated by the application of an increased ac voltage, but the response speed of the retarder in that direction is limited by the responsiveness of the liquid crystal cell material. Second, a change of retardance in the other direction must be effectuated by reducing the applied voltage and allowing the liquid crystal material to relax back to a new retardance; that is, it cannot be driven by the application of a voltage. These two drawbacks greatly limit the response speed of a liquid crystal cell retarder and, therefore, the applications to which the retarder may be put.
In particular, the slow response time of known liquid crystal cell retarder systems limits the speed with which they can switch between intermediate values, and corresponding polarization states, over a wide range of retardances. This limits their effectiveness in producing rapid changes in light polarization in optical instruments and laboratory set-ups, and in controlling polarization and fiber-optic communication systems where significant polarization fluctuations may occur. Accordingly, there is a need for a liquid crystal cell retarder system which provides a faster response time.